1. Field of the Invention
The subject invention generally relates to a mixing assembly for mixing bone cement.
2. Description of the Related Art
The application of bone cement to a bone during surgical procedures, such as the attachment of a prosthesis or pathological fracture fixation, is well known in the surgical community. With regard to the attachment of a prosthesis, the cement is packed into the bone and the prosthesis is then attached. The cement cures and a bond develops between the bone and the prosthesis. Other uses of bone cement include repairing or mending bone fractures or shattered bone occurring from extreme trauma. Bone cement may also be used during cosmetic or dental surgery. Moreover, bone cement may be used as a drug delivery or release system, whereby the bone cement is mixed with antibiotics or other desired drugs and applied to a specific surgical site such that the drugs leach out and are delivered directly to the surgical site. Some bone cements are also designed to be absorbed by the body over time.
Typically, the bone cement is prepared by thoroughly blending two components. Typical bone cement mixtures comprise a powdered polymer or copolymer, such as a polymethylmethacrylate, and a liquid monomer, usually a methylmethacrylate. Conventionally, the combining of the powder and liquid components is carried out using a container and a spatula resulting in the formation of a quick setting bone cement material. Because of its quick setting nature, the bone cement is usually prepared in the surgical room in conjunction with the surgical procedure. Once the bone cement is thoroughly mixed, the surgeon promptly removes the necessary amount of cement, inserts it into a delivery device or manipulates it by hand, and applies it to the appropriate surface or cavity before the cement mixture cures or hardens. However, there are a number of disadvantages to this method of mixing bone cement.
First, combining the monomer liquid and polymer powder causes noxious fumes to be emitted. Thus, it is desirable to prevent these fumes from escaping into the atmosphere. Second, the cement ingredients must be mixed quickly, thoroughly and uniformly to maximize homogeneity while reducing or eliminating the formation of air bubbles to impart high mechanical strength and bonding properties to the bone cement. Inherent in the mixing process, air bubbles are generated in the mixture from air residing in the powder and in the mixing container. Moreover, air bubbles are produced when the monomer vaporizes to produce a gas during the mixing process.
To evacuate the maximum amount of air and gas entrapped in the container and mixture, it is known in the art to perform the mixing in a mixing chamber under vacuum. Further, various devices are available wherein cement may be mixed under vacuum. Such devices include a mixing chamber coupled with a dispensing syringe connected to a vacuum source. Although such devices are efficient and clean, they are expensive and inhibit application of the cement compound by hand, which may be the preferred or necessary method in a given procedure.
Alternatively, mixing assemblies, i.e., cement mixing bowls, are also known in the art. Such mixing bowls generally include a housing and a lid defining a mixing chamber with a mixing paddle extending into the housing and disposed in the mixing chamber. Typically, the mixing paddle is rotated, i.e., driven by a handle extending out of the lid. In addition, the mixing chamber is generally connectable to a vacuum source for creating a vacuum within the mixing chamber.
Such mixing bowls are disclosed in U.S. Pat. Nos. 5,494,349 and 6,254,268. These mixing bowls are deficient for a variety of reasons. Overall, these mixing bowls do not provide adequate mixing and do not permit establishment of a sufficient hermetic seal. For example, the mixing bowl disclosed in the '349 patent to Seddon does not provide adequate mixing of the monomer liquid and polymer powder. More specifically, the single mixing paddle disclosed in the '349 patent, which only rotates in one direction, does not adequately mix the monomer liquid and the polymer powder. Ultimately, inadequate mixing of these two components results in a bone cement that lacks the required mechanical strength and bonding properties. The mixing bowl disclosed in the '349 patent is also deficient because the mixing paddle is not adequately biased against an interior wall of the housing, specifically a bottom interior wall, to sufficiently scrape the bone cement from the interior wall. As a result, excessive air and gas bubbles can remain present in the bone cement, which is detrimental to various properties of the cement. Instead, to have this mixing paddle adequately biased against the interior wall of the housing, the lid either cannot be tightened about the housing such that a completely hermetic seal is not established, or the lid has to be tightened too much about the housing such that the completeness of any hermetic seal is sacrificed. More specifically, the lid in the '349 is particularly susceptible to deflection when the mixing bowl is under vacuum. This deflection may ‘pinch’ the mixing paddle at the bottom interior wall such that increased torque is required to mix the bone cement. When the mixing bowl of the '349 patent is not under vacuum, there is typically a large gap between the mixing paddle and the interior wall of the housing. The mixing bowl of the '349 patent is further deficient in that it does not strategically incorporate a gear set that provides for more than one mixing paddle and for more than one rotational direction for optimum mixing of the bone cement. The mixing bowl disclosed in the '268 patent to Long realizes theses same deficiencies.
Due to the deficiencies associated with the mixing assemblies of the prior art, including those described above, it is desirable to provide a unique mixing assembly that solves one or more of these deficiencies.